In powder metallurgy by which a sintered body is manufactured using an iron powder or a copper powder as a main raw material, generally, there is used a mixed powder including a powder of the main raw material, a sub raw material powder (such as a graphite powder or an alloy component) for improving the physical properties of the sintered body, a lubricant, and the like. Particularly, in order to improve the mechanical physical properties (such as strength and hardness) of the sintered body, generally, carbon supplying component (carbon source) such as graphite is added, and the mixture is formed, followed by diffusion and carburization of the carbon source into the iron powder during a heat sintering treatment.
However, a graphite is smaller in specific gravity and smaller in particle diameter than an iron powder. For these reasons, mere mixing thereof results in that the graphite and the iron powder are largely separated from each other, and that the graphite segregates. Thus, uniform mixing thereof unfavorably cannot be achieved. With the powder metallurgy method, sintered bodies are mass-produced. For this reason, generally, a mixed powder is previously stored in a storage hopper. In the storage hopper, a graphite having a small specific gravity tends to segregate at the upper layer part of the hopper. Accordingly, when the mixed powder is discharged from the hopper, the concentration of the graphite increases in the end of hopper discharge. Thus, in the portion having a high graphite concentration in the sintered body, a cementite structure precipitates, resulting in the reduction of the mechanical characteristics. When a variation is caused in the content of carbon in the sintered body due to the segregation of graphite, it becomes difficult to manufacture components with stable qualities. Further, in the mixing step or the forming step, the segregation of graphite causes dust emission of the graphite powder. This unfavorably results in the problems of the aggravation of the workplace environment and the reduction of the handling property of the mixed powder. The foregoing segregation also similarly occurs not only for graphite but also for other various powders to be mixed with the iron powder. This has created a demand for prevention of the segregation.
In order to prevent the segregation and the dust emission of graphite, broadly classified three methods have been proposed in the related art. The first method is a method for adding a liquid additive such as tall oil to a mixed powder (e.g., Patent Documents 1 and 2). This method has an advantage of enabling manufacturing with simple facilities. However, when a liquid additive is added in an amount necessary for the segregation preventive effect to be observed, a liquid cross-linking force acts on among iron powder particles. This unfavorably results in extreme aggravation of flowability. The second method is a method in which a solid binder such as a high molecule polymer is dissolved in a solvent, and is uniformly mixed therein, followed by the evaporation of the solvent, thereby to allow graphite to adhere to the surface of an iron powder (Patent Documents 3, 4, and the like). This method has advantages of being capable of surely allowing graphite to adhere thereto, and also having a wide choice of options for lubricants to be used. However, the flowability of the mixed powder may be insufficient according to the composition. The third method is a so-called hot melt method characterized by heating and melting a relatively lower molecular weight lubricant such as fatty acid during mixing with an iron powder (e.g., Patent Document 5). The molten lubricant is uniformly fixed on the iron powder surface. For this reason, the temperature control during mixing is very important. Further, there is also a deficiency that the options for usable lubricants are restricted. With any of the first to third methods, an organic binder is added, which must result in a complicated step. This has created a demand for a more simple method.
Incidentally, although irrelevant to the segregation prevention, there is also proposed a technology of controlling the particle size of graphite. In Patent Document 6, a 0.1- to 2-μm graphite and an iron powder are mixed in a vibration mill with adding additives in a specific atmosphere such as ammonia. Thus, the iron powder particle surface is covered with graphite particles. In Patent Documents 7 and 8, the particle size of graphite is controlled, and using an organic binder, the iron powder surface is covered with graphite.